There are several isotopic methods and analyses of trace gases available to determine the age of groundwater. A competent estimation of age requires the inclusion of the boundary conditions like location and environment of the water resource, hydrogeological/geological cross sections, hydraulic characteristics, hydrochemical composition, supraregional studies and former investigations of the recharge area.
Depending on the age composition of the groundwater, environmental isotopes cover a wide range of time and scale from days to several millennia.
Very young groundwater recharge in the range of weeks, months or even a few years can be analysed over the compilation of time series of the stable isotopes oxygen-18 and deuterium.
For the determination of the mean residence time of young groundwater, recharged within the last 60 years, the radioactive isotopes tritium (3H) and krypton-85 (85Kr) are very useful. By simultaneous determination, 3H and 85Kr provide a possibility to quantify the age and fraction of the groundwater younger than 60 years.
The radioactive environmental isotopes (3H, 85Kr, 14C, 39Ar) mark surface waters on a global scale. Anthropogen activities are the main source for radioactive environmental isotopes besides cosmic radiation in the upper atmosphere. The greater part of 3H originates from nuclear weapon tests whereas 85Kr is discharged from nuclear facilities. The input into the groundwater happens directly via precipitation (3H) or via dissolved soil gas in percolating water (85Kr, 39Ar, 14CO2).
Due to longterm measurements at varous precipitation stations, the input of the environmental isotopes via the precipitation is well known. However, major local differences exist.
Radioactive environmental isotopes can also be used to estimate the age of old groundwaters in the range of hundreds to thousands of years. Primarily, the determination by carbon-14 (14C-DIC) in combination with carbon-13 (δ13C-DIC) has to be mentioned. By a simultaneous determination of 39Ar and 14C, a contribution of mixtures can be recognised and quantified.
For the groundwater age range from thousand to ten thousand years, isotopes and gas parameters can be used as indirect analysis method indicating the climatic conditions of the recharge. This can be achieved by the determination of the stable isotopes oxygen-18 and deuterium or by measuring the noble gas temperature. This way, groundwater recharge during cold periods (Pleistocene) can be differentiated from recharge in warm periods (e.g. Holocene).
In case of groundwaters of high ages, the determination of the gas isotopes 3He/4He and 36Ar/40Ar are applied as indirect analytical methods.
Very high groundwater ages can be estimated by the activities of the radioactive isotopes chlorine-36 and krypton-81.